Sorry bruv, most of the websites I used to buy blowgun gear from either dont exist anymore, or dont carry much in the way of .62 cal gear.
I see several DIY approaches to a good mouthpiece, but nowhere to just buy one. The type of mouthpiece you show is very, very common for .40 and .50 cals, but Ive only ever seen them a couple of times for a 62.
Kinda makes me wish I had a 3D printer cuz there's all kinds of gadgets could be dreamt up and built with one of those things!!
Good luck, and if ya find a place, lemme know, cuz my 3 favorite sites are now basically dead (for .62 cal, .40's and .50's a plenty, but they just aren't my jam after using big bores!)
Target zone sports is an awesome site, though, I go to check now, and of course everything is down for maintenance for the moment -_-
I think turning one on a lathe would be a great option, as I dont have a lathe right now I could just put my lathe chuck in my drill and vice that up to make one work. Not making something big, but I just got my Pro Cold Steel big bore, the 3mm tubing version, and I'm pretty [email protected] stoked about it!! But that handle/mouthpiece thing, more ergonomic or no, I just dont like them. I'd rather have those 10 speed city bike handle wraps where the mouthpiece ends, and one near the middle for my 'grips' and have a [email protected] mouthpiece I made myself!! Good call Ghost!